Spatial structure of 4nπ helicene dianions

Helicene dianions, e.g. phenanthrene derivatives, once believed to be non-helical, maintain their chirality as 4nπ dianions. Phenanthrene 5, as well as substituted phenanthrene derivatives, undergo a two-electron reduction to form the respective 4nπ dianion e.g., 5^2-/2Li⁺. Phenanthrene derivatives substituted at the 4- and 5-positions (bay substituents) e.g. 6-11, which are helical, afford stable dianions. These dianions are also prepared by a two-electron reduction of the (4n + 2)π electron hydrocarbons and show, in their ¹H NMR spectra, a quench of the paratropicity compared to 5^2- as well as a line shape dependence on their twist angle. The quench of paratropicity was also observed in the closely related charged helicenes derived from the benzo[g]chrysene system, i.e. anions 12^2- and 13^2-. The twist angles were calculated by MMX and MNDO calculations for the neutral systems, i.e., 5-11, and by MNDO for the dianions. MNDO calculations also included the preferred location of the counter cation. A dynamic NMR spectroscopic study proves experimentally the helicity of anion 11^2- thus shedding light on the behaviour of this novel class of dianions.